Operating in the Otto or the Diesel cycle on the high levels of temperatures and pressures of working field, a conventional piston reciprocating engine has fairly a high thermal efficiency. However, one has a number of well known disadvantages, which cannot be overcome in its present configuration, such as heavy weight, reciprocating unbalance, and insufficient utilization of capacity of the gases to expand resulting in not only considerable energy losses but also noise exhaust operations necessitating energy consuming mufflers. On the other hand, although a gas turbine engine, operating in the Brayton cycle on the comparatively low levels of temperatures and pressures of working fluid, has a low thermal efficiency, one has the advantages of low weight, no reciprocating unbalance and continuous power generation, in addition to having a higher power output than that of the diesel engine due to operating at higher RPMs. Now, suppose that a gas turbine and, for example, a diesel engine all operate at the same compression ratio, then the gas turbine thermodynamically has a higher thermal efficiency than the diesel engine. However, the gas turbine cannot operate at the high level of the same compression ratio as the diesel engine because it is difficult to cool the all elements in contact with high temperature working fluid in the turbine.
In internal combustion engines, the higher the compression ratio is, the higher the temperature of working fluid, and consequently the thermal efficiency, and generally, the higher the RPMs of an engine is, the higher the power output of the engine is. Thus an engine must operate at high compression ratio as well as high RPMs in order to achieve both the high thermal efficiency and high power output. Now, if an engine operates at high compression ratio, the temperature of working fluid correspondingly becomes high. Accordingly, the all elements in contact with the high temperature working fluid must properly be cooled so as not only to maintain a lubricant film on sliding surfaces of the elements, such as cylinder walls, but also to prevent destruction of the elements, such as cylinder heads, pistons and exhaust valves, due to overheating. Also, every rotor of the engine must be balanced in the moment of inertia about its axis to rotate at high speed.
Since the wankel rotary engine has developed as a commercial engine, several other rotary type combustion engines have been devised up to present. However, none of them resolve all of the problems of the said high compression ratio, proper cooling and rotor balance of the moment of inertia. Thus none of the devised rotary engines have realized their commercial use.
In otto cycle spark-ignition engines, a homogenous air/fuel mixture is rapidly deflagrated in a moment by electrical ignition means, which is generally assumed to be a constant-volume combustion process. In compression-ignition diesel engines, on the other hand, a heterogenous air/fuel mixture burns at comparatively slow speed, which is generally assumed to be a constant-pressure combustion process. Now, if the above two engines all operate at the same compression ratio, the spark-ignition engine with the constant-volume combustion must thermodynamically have a higher thermal efficiency than the compression-ignition diesel engine with the constant-pressure combustion. In the otto cycle spark-ignition engine, however, the compression ratio is limited to some maximum value, to preclude preignition of homogenous air/fuel mixture and combustion knock. This establishes a practical limit on the thermal efficiency of the engine.
The problem is resolved somewhat by the compression-ignition diesel engine. Thus the compression ratio, and consequently the efficiency, generally is higher than that of the spark-ignition engine. But because the system uses a heterogeous air/fuel mixture, the diesel engine operates at lower RPMs, and therefore has a lower power output.